Police have arrested 11 people in raids around Britain, as an ISIS-affiliated news agency claimed that the extremist group was behind the outrage.

Prime Minister Theresa May revealed the perpetrator was British born and once linked to violent extremism, in a statement to Parliament a day after it was locked down when the assailant breached its perimeter.

Four people died in the attack -- an American citizen, a woman of Spanish origin and a police officer stabbed as he defended Parliament -- and another 40, many of them from other countries, were injured. Of those, 29 required hospital treatment and seven are critical, police said.

May said the attacker had been investigated by security services but was regarded as a "peripheral figure." Authorities did not know he was about to mount an assault, she said.

Britain's most senior counterterror police officer said inquiries were continuing in London, Birmingham and elsewhere after the lone attacker plowed a car into crowds of people, killing two people, before stabbing a police officer dead in the grounds of the UK Parliament.

A tweet from ISIS-affiliated news agency Amaq said the attacker was "a soldier" of ISIS inspired by its message. However, ISIS has provided no evidence for any direct links to the assailant, whom it did not name.